Request manager

[gagarin55]

Is suggested there emeraldpay/emerald-wallet#475
The idea is:

Let's make two api functions, a standard one with direct call, and another one which will buffer requests into a batch call (JSON RPC batch operation) and execute it once 100ms, parse results and resolve original promises one by one.

-> call 1 -> [a]
-> call 2 -> [a, b]
-> call 3 -> [a, b, c]
...
100ms -> JSON RPC batch -> resolve a -> resolve b -> resolve c